A Conrail spokesman told NBC News that about 184,000 pounds of the chemical spilled out of one of the tanker cars and into the creek. A decontamination unit is at the scene.

A Coast Guard spokeswoman told Philly.com that a half-mile area around the bridge over Mantua Creek – which empties into the Delaware River just across from Philadelphia International Airport – has been evacuated.

Twitter/@HTSERTThe same bridge where today's derailment occurred had been repaired in 2009 after a collapse.

People in Paulsboro, West Deptford and East Greenwich Township have been notified of the potential danger and main roads have been closed.

At about 11 a.m. officials determined that air quality had improved enough to lift the lockdown on area schools because vinyl chloride evaporates quickly and becomes inactive when mixed with water.